Conventional secondary electrochemical cells include a plurality of plate-like positive and negative electrodes contained within a prismatic housing. The electrodes are aligned in an alternating positive/negative array and are typically separated by layers of frangible porous electrically insulative material facing their major side surfaces.
The electrochemically active material of the individual electrodes can be of any type suitable. Alloys of alkali metals or alloys of alkaline earth metals containing alloying materials of aluminum, silicon, magnesium and combinations thereof are used for the negative electrodes. The positive electrodes generally contain chalcogenides or preferably transition metal chalcogenides as electrochemically active material. Both the positive and the negative electrodes include an electrolyte, such as mixtures of alkali metal halides, mixtures of alkaline earth metal halides, or combined mixtures thereof. Cells containing these components and types of active materials and electrolytes are well known and described more fully in U.S. Pat. No. 4,313,259.
Electrodes of like polarity are interconnected by one of two busbars electrically coupled to the cell terminals, which extend through and are electrically insulated from the cell wall of the housing by feedthroughs. The electrical busbars are typically spaced lengthwise from one another within the cell housing and are each connected to individual electrodes by electrical conductors.
Full size batteries of this type are comprised of many cells grouped together in an end-to-end or face-to-face arrangement in a common battery housing and electrically connected in series to produce higher effective voltage output. Even a thin cell version is capable of very high current density.
The batteries are designed to operate at temperatures in the range of 375.degree.-500.degree. C. Existing battery designs, involving electrolytes normally fluid at cell operating temperatures, are concerned with electrolyte leakage past the separator between adjacent positive and negative electrodes. Leakage could consume the electrolyte by electrolytic decomposition and could produce metallic deposits sufficient to cause battery failure by shorting out the adjacent collectors or to the external battery housing.
In general, and with respect to typical cell designs, a positive feedthrough requires an electrical insulating seal. A negative feedthrough is part of the cell can, which is at the negative electrode potential. The positive electrode is, therefore, attached to the feedthrough, which must remain free of short circuits throughout the cell lifetime and must also prevent electrolyte escape from the cell. Feedthroughs currently used in lithium alloy/iron sulfide batteries are of a mechanical compression design, with a lower solid insulator of beryllia (BeO) and an upper insulator of alumina (Al.sub.2 O.sub.3 ). The actual seal component thereof, may be formed of a compressed bed of boron nitride (BN) powder, which is effective in preventing electrolyte escape, but is not fully hermetic.
Conventional ceramics such as MgO, BeO, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, BN and AlN, however, have substantial drawbacks, such as, very poor mechanical properties, including poor fabricability and limited chemical stability, when exposed to nonaqueous corrosive environments at high temperature. Furthermore, the melting point of the conventional ceramics is in excess of 2,000.degree. C., which poses significant processing problems. Further, the thermal expansion coefficient of such ceramics is often completely incompatible with adjacent metal components present. If bonding to metals is necessary, the metal component (the brazing agent) must wet the ceramic or an intermediate glass phase must be used. However, such conventional glasses are not chemically stable nor operable at higher temperatures normally encountered in highly corrosive environments of interest. If such unstable brazes, glasses, or ceramics are used, the resulting reaction products can cause formation of undesired electrical conductor materials, rather than remaining as the desired insulator material.
Resistance is another problem associated with previous cell and feedthrough designs. Resistance mapping measurements made for various state-of-the-art lithium alloy/iron sulfide cells showed that approximately 50% of the overall cell resistance was external to the electrode stack and due, specifically, to the interelectrode connections (busbars) and terminal to interelectrode connections. As a result, the power output of such cells was only 50% of the theoretical level.
The search for an efficient, effective feedthrough apparatus meeting the requirements outlined above has been an ongoing concern in the art. One approach, which has met with limited success, involved reducing interconnecting impedance by increasing the size of copper-cored cell terminals and using massive interelectrode connections. In such a fashion, total cell resistance was reduced and that external to the electrode stack was about one-third of that earlier observed.
However, increasing feedthrough and terminal size results in a corresponding enlargement of the cell. In some instances, the feedthroughs are relatively large (on the order of several inches) and necessary to contain the seal housing and powder. As a result, the seals and terminals extend considerably beyond the battery header, substantially increasing the overall size of a cell system. The increase in size, however, does not necessarily promote longer cell life without electrolyte leakage and short circuiting. Alternatively, restrictions on such compressive designs tends to limit terminal diameters to the extent multiple terminal pins may be required for high current load.
In summary, a considerable number of problems and concerns exist with respect to previous cell designs, most are related to inefficient battery construction and operation, and result from the type of feedthrough/seal apparatus currently used.